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I would call and talk to the partner (or whoever) who extended the offer. When I called to accept my offer, the partner said they would send a formal letter to me (to accept my acceptance? who knows), which I imagine is the last step in the process.

Call the partner you had the most contact with (or the hiring partner, if you met with him/her) to accept. Follow up with an email to recruiting. A few days letter you should get a letter confirming your acceptance.